Sterically hindered phenolic compounds, for example, tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxymethyl] methane, octadecyl 3-(3,5-di-tert-butyl-4-hydroxy-phenyl)propionate, triethylene glycol bis[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionate], 3,9-bis{2-[3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy]-1,1-dimethy lethyl}-2,4,8,10-tetraoxaspiro[5.multidot.5] undecane or the like have been used as an antioxidant for organic materials such as polyolefines.
These compounds are generally produced by submitting corresponding alkyl propionates and alcohols to transesterification, for which basic catalysts such as alkali metal hydrides, alkali metal alkoxides, alkali metal amides, dibutyltin oxide, zinc salts, calcium salts or the like are used, as disclosed in U.S. Pat. Nos. 3,644,482, 3,944,594, 4,228,297, 4,618,700, 4,739,102, 4,754,045, etc. By the use of these catalysts, however, the production of the hindered phenols as mentioned above requires a long reaction time and causes the reaction solution to be colored in brown, and accordingly, does not afford products commercially presentable. With Dibutyltin oxide is superior to the other catalysts in respect of the reaction time, but still is susceptible to coloration and after the reaction, the resulting solution is yellow or yellowish-brown Interception from air, or any other measure for the prevention of coloration is thus a big consideration. The catalyst is difficult to remove or cannot be removed completely, particularly in a purification process such as recrystallization, and consequently, it is difficult to suppress its content within the allowable concentration of products. A method of obtaining the intended products by distillation is also known. However, the distillation process requires a long time and a high temperature upward of 240.degree. C. with the result that the products are colored.
In the case of tetrakis[3-(3,5-tert-butyl-4-hydroxy-phenyl)propionyloxymethyl]methane, U.S. Pat. No. 4,618,700 discloses a process for transesterification wherein 4.6-6.0 moles of the propionates per 1 mole of pentaerithritol are used and the excess propionates are recovered. This reaction per se was advantageous in that it was terminated in a short period of time, but required a long time for the distillation to recover a large amount of the unreacted excess propionates. Eventually, it took nearly the same period of time as the case with the reaction wherein about 4.4 moles of the propionates were used.
Considering the fact that recycling of the recovered propionates is limited to two times because of accumulation of decomposed propionates and the fact that the distillation for the separation of the products brings about the afore-mentioned disadvantages, this method is not industrially feasible.
Heretofore, in preparing this tetrakisester compound, an unesterified trisubstitute of the formula: ##STR2## has been produced as a by-product. In order to prevent compound (A) from remaining since this is difficult to remove, Japanese Patent Publication 60-13015 (1985) provides and discloses a method for producing the tetrakisester compound wherein dipentaerithritol is added to pentaerithritol in an amount of 0.1-1.0% thereof, whereby the yield of compound (A) is reduced to less than 5%. This is also disadvantageous in that the particular substance must be added.
Hence, in order to produce sterically hindered phenol compounds useful as an antioxidant for organic materials with high quality and economically, such a catalyst is desired that fulfills the conditions: (1) the reaction solution after the termination of the reaction is not colored, (2) the removal of the catalyst is easy, and (3) no specific substance need be added to suppress the formation of byproducts.
The present inventors have investigated intensively with a view toward solving the problems noted above and accomplished this invention by finding that hindered phenol compounds can be produced by the use of monoalkyltin compounds as a catalyst without coloring, with the result that the removal of the catalyst is facilitated and the content of impurities is low.